Why are convertible bond announcements associated with increasingly negative issuer stock returns? An arbitrage-based explanation

While convertible offerings announced between 1984 and 1999 induce average abnormal stock returns of −1.69%, convertible announcement effects over the period 2000-2008 are more than twice as negative (−4.59%). We hypothesize that this evolution is attributable to a shift in the convertible bond investor base from long-only investors towards convertible arbitrage funds. These funds buy convertibles and short the underlying stocks, causing downward price pressure. Consistent with this hypothesis, we find that the differences in announcement returns between the Traditional Investor period (1984-1999) and the Arbitrage period (2000-September 2008) disappear when controlling for arbitrage-induced short selling associated with a range of hedging strategies. Post-issuance stock returns are also in line with the arbitrage explanation. Average announcement effects of convertibles issued during the Global Financial Crisis are even more negative (−9.12%), due to a combination of short-selling price pressure and issuer, issue, and macroeconomic characteristics associated with these offerings.